This invention relates generally to broadband communications systems, such as cable television systems, and more specifically to an optical transmitter and a method of transmitting reverse analog signals within the optical transmitter by a burst-mode technique.
A broadband communications system 100, such as a two-way hybrid/fiber coaxial (HFC) communications system, is depicted in FIG. 1. Such a system may be used in, for example, a cable television network; a voice delivery network, such as a telephone system; and a data delivery network to name but a few. The communications system 100 includes headend equipment 105 for generating forward signals (e.g., voice, video, or data signals) that are transmitted in the forward, or downstream, direction along a first communication medium, such as a fiber optic cable 110. Coupled to the headend 105 are optical nodes 115 that convert the optical signals to radio frequency (RF) signals. The RF signals are further transmitted along a second communication medium, such as coaxial cable 120, and are amplified, as necessary, by one or more distribution amplifiers 125 positioned along the communication medium. Taps 130 included in the communications system split off portions of the forward signals for provision to subscriber equipment 135, such as set-top terminals, computers, telephone handsets, modems, and televisions. It will be appreciated that only one fiber link connecting the headend 105 with a node 115 is shown for simplicity; however, there are typically several different fiber links connecting the headend 105 with several additional nodes 115, amplifiers 125, and subscriber equipment 135.
In a two-way system, the subscriber equipment 135 can also generate reverse electrical signals that are transmitted upstream to the headend equipment 105. Such reverse signals may be amplified by any one or more of the distribution amplifiers 125 and converted to optical signals by the optical node 115 before being provided to the headend equipment 105.
Conventionally, an analog communications system transmits and receives the forward and reverse signals in the analog domain. An example of detailed optical paths including a headend and optical nodes that are suitable for use in an analog broadband communications system 200 is shown in FIG. 2. A headend 205 generates and transmits optical signals via optical transmitters 210a-n downstream through their respective fiber links 215a-n. It will be appreciated that there are a plurality of optical transmitters 210a-n transmitting optical signals to a plurality of nodes 220a-n, where each node 220 services a different pocket of the system depending upon the network design. Within the nodes 220a-n, an optical receiver 230a-n converts the optical signals to electrical signals. A diplex filter 235a-n then isolates the forward electrical signals from the reverse path and provides the electrical signals to coaxial cable 240a-n for delivery to the subscriber equipment 225a-n. 
In the reverse path, electrical signals emanating from subscriber equipment 225a-n are transmitted upstream via the coaxial cable 240a-n to the node 220a-n. The diplex filter 235a-n isolates the reverse signals from the forward path and provides the signals to an optical transmitter 245a-n for conversion of the electrical signals to optical signals. The optical signals are then transmitted upstream, via an optical fiber 248a-n to an optical receiver 250a-n located within the headend 205 for further processing.
If additional subscriber homes are added to the network 200, it may be necessary to add an additional node 220 that includes separate links for the forward and reverse path to address the additional subscriber equipment. Additionally, if the operator chooses to optimize the network 200 to accommodate an increase in the amount of reverse signals being transmitted by one optical transmitter, an operator can accomplish this by decreasing the amount of subscriber homes that a node 220 services. For example, an operator can reduce an existing network that includes 2000 subscriber homes per node to 500 subscriber homes per node, and add three additional nodes to the network. It can easily be understood that increasing the size of the network requires a significant amount of equipment and fiber.
It will be appreciated that separate reverse fiber paths, or links, are required because reverse optical signals cannot be combined like reverse electrical signals. More specifically, those skilled in the art will appreciate that when the light from multiple outputs of optical transmitters, where each output has a specific wavelength, is applied simultaneously to an optical receiver, intermodulation distortion results. If the differences between these received wavelengths are sufficiently small, the intermodulation distortion produced in the optical receiver will obscure the desired electrical signals, which are signals from 5 MHz to 42 MHz, at the output of the optical receiver. The drift in wavelength encountered in conventional optical transmitters makes this condition likely to happen.
Recently, new broadband applications, such as interactive multimedia, Internet access, and telephony, are increasing the amount of reverse signals within the reverse bandwidth. As a result, network operators are redesigning the networks 100 to effectively increase the total reverse signal carrying capacity, for example, by digitizing the reverse analog signals and allowing more signals to be transmitted within the existing reverse bandwidth. More specifically, a simplified digital reverse path that is suitable for use in a broadband communications system to digitize analog signals is depicted in FIG. 3. Digitizing the optical signals as shown in FIG. 3 allows the operator to increase the reverse signal carrying capacity that is demanded by the growing number of customers and interactive applications. Briefly, a plurality of digital transmitters 305a-n each including an analog-to-digital (A/D) converter 308a-n receives analog electrical signals from a number of connected subscriber equipment and converts the analog electrical signals to digital optical signals. Linked, via fiber optic cable 309a-n, to each digital transmitter 305a-n is a digital receiver 310a-n that includes a digital-to-analog (D/A) converter 315a-n and which is located further upstream in the network 300. The D/A converter 315a-n converts the received digitized optical signals back to analog electrical signals for delivery to the headend and further processing. An example of a similar digital reverse path is discussed further in commonly assigned, copending patent application Ser. No. 09/102,344, filed Jun. 22, 1998, in the name of xe2x80x9cDigital Optical Transmitterxe2x80x9d the disclosure of which is incorporated herein by reference.
Digitizing the reverse path, however, is an expensive technique to employ and most network operators may not be ready or able to invest in the required capital costs. Typically, network operators that have been operating for a substantial length of time do not have the digital equipment, such as digital transmitters and receivers, required to digitize the reverse signals. In order to accomplish this, the operators would have to substantially upgrade their system to include the digital equipment and may also have to lay extensive routes of fiber. The majority of operators have historically transmitted and received analog signals over an analog HFC system; therefore, due to the expensive undertaking of sending digital reverse signals, most operators would like an intermediate step to enable the efficient, low-cost delivery of reverse signals over their existing HFC system.
In summary, what is needed are devices and networks that are capable of transmitting and combining reverse optical signals, similar to the combining of reverse electrical signals, in order to ensure the reverse bandwidth is able to accommodate the increasing amount of reverse signals.